A conventional CRT creates an image on a phosphor coated display screen by focusing a beam of electrons onto the screen and thus making the phosphors emit visible light. The electron beam is typically formed by first causing a cathode inside the vacuum of the CRT to emit electrons by applying a differential voltage between the cathode and a grid. A CRT typically includes four grids including a control grid (referred to as Grid 1 or G1). The CRT may have one or more anodes to attract, focus, and accelerate the electrons. Thus, the anodes may collimate the electrons into a tight beam hitting a spot of the display screen. By creating currents in coils outside the CRT, a magnetic field is created within the CRT to direct the electron beam at any desired spot of the display screen.
Typically, the image is displayed on the CRT by scanning the electron beam across the screen, from left to right. When the end of each line is reached, the electron beam moves from right to left to reach the beginning of the next line, a process called horizontal retrace. Once all of the lines on the screen are traced, the electron beam moves from the bottom of the screen to the top of the screen during a vertical retrace interval.
During both the horizontal and vertical retrace intervals, blanking may be performed. During blanking, the electron gun is biased at a potential such that the electron beam is cut off or the retrace lines are invisible. Blanking is typically performed by applying a large negative voltage to G1 during the retrace interval.
In order to provide a large negative voltage to G1, a logic signal must be converted into a signal with a significantly larger voltage swing. This is typically accomplished by employing a linear amplifier to amplify the logic signal.